ZHANG Chunling,SI Wen,XIE Chunhu

1.College of Marine Sciences,Shanghai Ocean University,Shanghai 201306,China；

2.College of Oceanic and Atmospheric Sciences,Ocean University of China,Qingdao 266100,Shandong province,China

Abstract:The climate distribution and variation characteristics of temperature and salinity i n the upper surface layer of Sulawesi Sea(117-127°E,0-8°N)were analyzed using Argo T/S profiles of 0-1 500 m and XBT temperature profiles obtained by Chinese Antarctic scientific investigation.The results show that the temperature and salinity are respectively about 2.5 °C-30 °C and 33.2 ‰-35.1 ‰ in the Sulawesi sea.Compared with the vertical changes,the T/S horizontal gradients are smaller.The temperatures gradually reduce with the increase of the depth.The salinities increase firstly,then decrease,and increase finally with the increase of depth.The salinity has two lower and one higher salinity areas.The whole sea surface shows the characteristics of high temperature and low salinity.T he subsurface temperature is slightly lower with salinity increasing.Middle depths are characterized by high temperature and high salinity.At the depth of 500 m,the temperature and salinity tend to be uniform.And at the bottom,T/S represent lower temperature with higher salinity.Moreover,an obvious thermocline exists at the depth of 50 m-150 m.The thermocline depth in summer is less than 90 m and is about 110 m in winter.The thermocline exists in the deeper depth in April than that of November by XBT observation sections.In the central of Sulawesi Sea,the thermocline is also relatively deeper.

Keywords:Sulawesi Sea,temperature and salinity,Argo,XBT

1 Introduction

Sulawesi Sea,also known as Celebes Sea,is located on the edge of the tropical western Pacific.lt is a sea surrounded by Sulu lslands,Mindanao,Kalimantan,Sulawesi and Sangihe lslands(Fig.1).Meanwhile,it is connected with Sulu Sea in the northwest and enters Java Sea and Bali Sea in the southwest through the Makassar Strait,the southeast leads to the Maluku Sea through the Maluku Strait.The special geographical location makes the Sulawesi Sea an important water area connecting Southeast Asian countries and leading to the Pacific and lndian oceans.The seabed is deep and flat,with most wa ter depths of more than 2 000 m and the maximum water depth of more than 8 000 m.Having the richest plate subduction system in the world[1],it is recognized as a biodiversity intensive area in the scientific community.From the perspective of atmospheric circulation,the Sulawesi Sea is located at the intersection of walker circulation,zonal and meridional monsoonal circulation.lt is closely related to the Asian-Australian monsoon system,El Nino-Southern Oscillation(ENSO),lndian Ocean Dipole(lOD)and other sea-air coupling systems with globalor ocean basin scale[2,3].From the perspective ofocean circulation,the Sulawesi Sea is the bottleneck connecting and affecting the circulation and water mass mixing movement of the western Pacific Ocean and the Eastern lndian Ocean.lt is the formation area of the lndonesia through-flow and controls the balance of heat and fresh water in the lndian Ocean along the global thermohaline cycle[4,5].

Fig.1 The topography of the Sulawesi Sea

The Western Pacific marginal tectonic belt is the largest and most complex plate boundary on earth,which has always been the focus of Geoscience.Many scholars have done a lot of research on it from different aspects such as plate age[6,7],plate expansion center position[8,9],structural characteristics and evolution[10-12],and revealed the changes of seabed topography and geological environment in Sulawesi Sea and its surrounding waters.The Sulawesi Sea,located at the western end of the tropical Pacific,is a gathering place where the variability of various sources in the ocean spreads westward,and its ocean circulation is also affected by many factors.The strong air-sea interaction around the oceanic continents makes the Sulawesi Sea subject to strong monsoon for a long time and Madden-Julian oscillations[13].In addition to atmospheric wind forcing,the sea is also at the intersection of equatorial and coastal waveguides of the Pacific Ocean,lndia and adjacent marginal seas,so that it is also affected by ocean changes far away.Moreover,the unique geometry(approximate rectangle)of Sulawesi basin can amplify ocean changes through resonance[14-16].

Through site observation,numerical simulation and theoretical research,predecessors have carried out a series of studies on the dynamic impact of Sulawesi Sea and its adjacent waters on the global climate system and air sea interaction[17],which hassignificantly improved our understanding of the seasonal or longer-term scale change of local circulation.As two very important basic physical parameters in oceanography,temperature and salinity are closely related to many important phenomenon in physical oceanography.Wang,et al.(2018)analyzed the north-south difference in the properties of water masses in the Lembeh Strait of Sulawesi Sea in northern lndonesia by using the CTD observation data in September 2015 and September 2016[18,19].Chen,et al.(2018)also mentioned that Sulawesi Sea may have unique water mass characteristics when using satellite data to explore the dynamic mechanism of interannual oscillation there[5].However,as Sulawesi Sea is a relatively closed sea,there are few internal and deep-seated field observations,and most studies focus on the surface of its marginal sea.

In this paper,the temperature and salinity characteristics of the upper layer(0 m-1 500 m)in the Sulawesi Sea(117-127°E,0-8°N)are systematically described and analyzed by comprehensively using the observation profile data of Argo(Array for Real-time Geostrophic Oceanography)and the temperature profile data obtained by XBT(expendable Bathy Thermograph)probe put through the sea during the Chinese Antarctic scientific investigation.lt provides a systematic theoretical basis for a comprehensive understanding of the physical environment characteristics of the sea and an in-depth discussion of the dynamic mechanism of marine phenomena in the future.

2 Data and processing

In this paper,two kinds of observation data were used:using Argo climate data,the steady normal distribution of environmental parameters of each water layer was analyzed;based on the XBT profile obtained from the Antarctic scientific expedition,the instantaneous characteristic differences of environmental parameters on the observed section are discussed.

2.1 Argo data

Due to the special geographical location of Sulawesi Sea,it is difficult to obtain the ce ntral and deep data by conventional observation.The number of temperature and salinity profiles obtained by the international Argo project in the global sea is increasing at the the the the rate of more than 140 000 profiles every year,which far exceeds other conventional observation methods.But even Argo automatic profile floats have far less real-time observations in this area than in the open ocean.During the 16 years from January 2004 to the end of December 2019,the total number of effective Argo temperature/salinity profiles in Sulawesi Sea is 2 039(Fig.2).Most of these profiles are located in the middle of the sea with water depth greater than 2 000 m,while there are few profiles at the edge of the island or at shallow water depth.The original profile is available at ftp://ftp.argo.org.cn/pub/ARGO/global/[20].Therefore,based on these data,this paper,after qualitycontrol[12,21],and referring to the production method of BOA_Argo data set[22]of Argo Real-time Data Center in China,constructed the Argo multi-year climate state network Gwen and salinity data of Sulawesi Sea with the horizontal resolution of 0.25×0.25° and vertical resolution of 37 layers(within the range of 0 m-1 500 m)by using the improved Barnes(1973)successive correction analysis[12,23,24].And using the maximum angle method[25,26],the depth of the mixing layer is calculated,ignoring the influence of the barrier layer and the compensation layer.

Fig.2 Argo profile observation station

2.2 XBT data of Antarctic scientific research

The XBT temperature observation profile data obtained durin g the 28th-32ndAntarctic scientific research missions carried out by Shanghai Ocean University aboard R/V Xuelong"from 2011 to 2015 were used,and the observation profile corresponding to the Sulawesi Sea in five voyages was selected to analyze the instantaneous characteristics of temperature.These observation profiles have passed the unified quality control of Polar Research lnstitute of China.Among them,the XBT stations of the 28thand 29thAntarctic scientific expedition are mainly concentrated in the Southern Ocean.There are 14 stations in the Sulawesi Sea in the 30thAntarctic scientific expedition.Here,7 profiles(blue pentagram in Fig.3)longitudinally across the Sulawesi Sea are selected as section A,and the observation time is November 12-13,2013.There are 6 stations in the 31stAntarctic scientific expedition(red triangle in Fig.3),from April 2 to 3,2015,set as section C.The 32ndAntarctic expedition has 24 stations in the sea.According to the observation time,it is divided into 14 stations of section B(black circle in Fig.3)and 10 stations of Section D(yellow box in Fig.3),with time spans of November 11-12,2015 and April 3-5,2016,respectively.

Fig.3 XBT observation station for 30-32 Antarctic scientific surveys

3 Analysis of environmental characteristics

3.1 Temperature

Firstly,we analyze the temperature characteristics of the upper layer(0 m-1 500 m)of the Sulawesi Sea using the objective Argo temperature data.In Fig.4,six representative water layers(surface layer,50 m,150 m,300 m,500 m and 800 m)to give the large-scale distribution of temperature in Sulawesi Sea.It can be seen from the figure that from the surface layer to 800 m,the temperature change at each water layer is not obvious,while the vertical temperature gradient is relatively large.The surface temperature(Fig.4a)is basically between 29.2 °C-29.5 °C.The temperature of Sulu Islands and Mindanao Islands is slightly lower in the northeast of the sea,and gradually increases from the northeast to the southwest.The 50 m layer(Fig.4b)also shows a temperature change trend similar to that of the surface layer.From the southwest edge of Medan Islands to the Makassar Strait,the temperature gradually increases from 27.2 °C to 28.5 °C,and the temperature change range of the whole sea is about 1.3 °C.The temperature of 50 m layer is slightly lower than that of the surface layer,but the horizontal temperature gradient increases significantly.The temperature at 150 m decreased significantl(Fig.4c),and the temperature range is about 18.8 °C-19.8 °C,still showing a trend of low temperature in northeast and high temperature in southwest.At 300 m layer(Fig.4d),the temperature continues to decrease to 10.4 °C-11.2 °C,but the high temperature area obviously moves to the north,showing the distribution characteristics of high in the northwest and low in the southeast.Meanwhile,this feature also exists at 500 m and 800 m(Fig.4e and Fig.4f),but the temperature gradually becomes uniform,and the temperature variation range of the whole sea is about 0.5 °C.

Fig.4 Temperature climate state distribution of surface layer(a),50 m(b),150 m(c),300 m(d),500 m(E)and 800 m(f)in Sulawesi Sea

In order to display the vertical distribution characteristics of temperature more clearly,Fig.5 shows the cross-sectional distribution of climatic temperature along 123°E longitude and 3°N latitude.Obviously,the temperature of both meridional and zonal sections decreases gradually with the increase of depth,and at the depth of 50 m-150 m,the temperature contour is particularly dense.The thickness is less than 100 m,the temperature decreases from 28 °C to about 18 °C,and there is an obvious thermocline.With the increase of depth,the vertical gradient of temperature also decreases,and the temperature is nearly uniform in the deep layer below 1 000 m.On the section along the 123°E longitude(Fig.5a),for the upper surface layer shallower than 100 m,the isoline tilts upward from south to north,that is,the temperature in the northern sea is higher than that in the southern sea at the same depth.This is related to the mixing of near surface seawater brought by the high latitude low-temperature water along the coast of Mindanao flowing into the Sulawesi Strait from the northeast[5].At the depth of 250 m-1 000 m,due to the influence of topography,the water exchange between the sea and the Western Pacific is weakened,the temperature isoline tilts downward from south to north,and the temperature generally presents the characteristics of high in the south and low in the north.On the section along the 3°N(Fig.5b),from the surface layer to the bottom layer,the isoline basically tilts from west to east,that is,the temperature in the west of Sulawesi Sea is higher than that in the east,which is consistent with the trend shown in Fig.4.

Fig.5 Climatic temperature profile distribution along 123°E longitude(a)and 3°N latitude(b)in Sulawesi sea

In Fig.6,four temperature sections(shown in Fig.3)are drawn by using XBT temperature profiles obtained from three Antarctic scientific research expeditions.Among them,the observation stations A1-A7 and B1-B14 in Section A and section B are XBT observations conducted by R/V Xuelong from Shanghai to Antarctica in November 2013 and 2015 respectively.The XBT observation stations(C1-C6 and D1-D10)of section C and Section D are the observation conducted by R/V Xuelong from south to north on the way back from Antarctica to Shanghai in April 2015 and 2016.Here,A1,B1,C1 and D1 are set as the starting points of the four section distances.

In April,the temperature of the Sulawesi Sea at a depth of 50 m(Fig.6c and Fig.6d)basically tends to be uniform,and there is an obvious upper mixing layer,while the depth of the upper mixing layer in November is relatively shallow(Fig.6a and Fig.6b).ln particular,the temperature observation profile(A2-A5 and B6-B10)in the middle of the sea,the upper mixing layer rises to the sea surface,and the depth of the upper mixing layer in November 2015 is slightly shallower than that in 2013.ln April,except for D8 and D9,other observation stations are located in the Sulawesi Sea(Fig.3).The upper mixing depth of these temperature profiles is about 50 m,which is slightly shallower in 2016 compared with 2015.Within the depth range of 100 m-150 m,the isotherms are relatively dense,which is more obvious in November than in April,and the thermocline depth in November shows a trend of gradually deepening from north to south.ln the southern sea in November 2013,the thermocline depth exceeded 150 m,and the thermocline depth in April is also generally deep in the south and shallow in the north.The thermocline depth in 2016 is slightly shallower than that in 2015,but the temperature gradient is obviously large.With the increase of depth,the vertical temperature change of each observation section decreases gradually.The shallow isotherms of section A and Section D at 300 m are inclined downward from left to right(the temperature is lower in the north and higher in the south),while the shallow isotherms of section C and Section D are obviously inclined upward from south to north,which also shows the characteristics of temperature in the south and lower in the north of the water layer,which is consistent with Fig.4 and Fig.5.

It is worth noting that there is a relatively strong disturbance at a shallow depth of 150 m in Fig.6d(Section D).The three observation points D1-D3 are located in the southwest,close to the Makassar Strait,and their temperature section sinks obviously,while the three observation points D8-D10 are located in the northeast,close to Mindanao lsland,and the isotherm is tilted up.On the observation stations D4 and D6,the temperature is kept at relatively high of 8 °C from 500 m to 750 m.This seems to be related to the topography,surface flow field and the mixing of upper seawater caused by it.

Fig.6 Temperature profile distribution of XBT station in Sulawesi Sea.

3.2 Thermocline

With the help of real-time Argo temperature profile,the scatter distribution of the upper boundary depth of the thermocline calculated by the maximum Angle method[26]is shown in Fig.7.The influence of the Barrier layer and the compensation layer is ignored in this paper,and the upper boundary depth of the thermocline here can also be called the depth of the mixing layer.Summer(July to September)is a strong period of thermocline in the northern hemisphere ocean.The thermocline depth of most observation stations in the Sulawesi Sea is less than 150 m(Fig.7a).Most of the sites in the waters near the Sulu Islands in the northwest are shallow at 100 m,and the central sea is about 80 m-120 m.The thermocline depth of some sites reaches 250 m,which may be related to the observation time of Argo profile.The upper boundary depth of the thermocline in the waters near the eastern Sangyihe lslands and Karkalalon lslands is relatively deep,mostly more than 100 m.The depth of the upper boundary of the thermocline in winter(January to March)is relatively deep(Fig.7b),most of which are more than 100 m,the middle sea is mostly between 150 m-200 m,and the sea near the boundary is relatively shallow,about 90 m-120 m.

Fig.7 Dispersion distribution of thermocline depth in Sulawesi Sea in summer(a)and winter(b)

The corresponding frequency statistics show that the thermocline depth is divided into a depth interval of every 10 m.In summer(Fig.8a),there are 480 Argo profiles in the Sulawesi Sea,and the maximum thermocline depth does not exceed 280 m.Among them,the depth range of the upper boundary of the thermocline with the largest frequency is 60 m-70 m and 80 m-90 m,with 56 profiles respectively,followed by 90 m-100 m and 53 profiles.There are about 320 profiles with thermocline depth between 60 m-120 m(frequency>30 in each interval),accounting for 67% of the total observation number,while the number of observation profiles with thermocline depth greater than 120 m and less than 50m are 122(about 25%)and 42(8%)respectively.ln contrast,among the 519 profiles in winter(Fig.8b),the thermocline depth with the largest frequency ranges from 100 m to 110 m,with 64 profiles,and 315 profiles with the depth range of 80 m to 160 m(the frequency of each interval exceeds 35),accounting for about 61% of the total profiles,while the thermocline depth less than 80 m and 160 m to 290 m account for about 22% and 17% respectively.lt can be seen from the statistical results of the total amount for many years(Fig.8c),the upper boundary depth of thermocline in Sulawesi Sea is concentrated in the range of 60 m-140 m(accounting for about 72% of the total sections),and the number of profiles is more than 100 for every 10 m depth interval.Among them,the frequency corresponding to the thermocline depth of 80 m-110 m is higher(>200 profiles).lt can be seen that although the real-time thermocline depth reflected by Argo observation is different due to its temporal and spatial characteristics,its multi-year average thermocline depth is about 100 m,which is consistent with the thermocline depth shown in Fig.5.

Fig.8 Statistics of upper boundary depth frequency of thermocline in summer(a),winter(b)and whole year(c)

3.3 Salinity

Since there are few station and satellite observation data on the salinity of Sulawesi Sea,only Argo salinity climate state data are used to describe and analyze the upper surface salinity characteristics of this sea.As shown in Fig.9,similar to temperature,the horizontal difference of salinity is small,while the salinity value between vertical layers is quite different.The salinity range of the surface layer(Fig.9a)is 33.5-34.0‰.The salinity of the sea near Kalimantan island in the southwest is obviously low(<33.8‰),and the salinity value gradually increases from west to East.The salinity values of the sea near Mindanao Island in the middle and North of Sulawesi Sea is about 34.0‰.In the 50 m layer(Fig.9b),the salinity level distribution still shows the trend of low in the southwest and high in the north central part,but the overall salinity value increases compared with the surface layer.The minimum salinity of 50 m in the whole sea is about 34.0‰,the maximum salinity value exceeds 34.3‰,and the horizontal gradient of salinity in the whole sea decreases.Similar to the temperature,the near surface layer is greatly affected by the ocean current.At 150 m(Fig.9c),the mixing effect is weakened and the salinity distribution characteristics are changed:the middle part shows a high salinity center(>34.73‰),which almost occupies most of Sulawesi Sea,while the salinity value of the marginal sea near the islands in the northwest and southeast is relatively low.The salinity value of 300 m layer is reduced to below 34.5‰(Fig.9d),and the horizontal gradient of the whole sea is reduced,the maximum salinity difference is no more than 0.05‰,and the salinity value of most sea areas is between 34.39-34.40‰.More than 500 m deep(Fig.9e and Fig.9f),the salinity distribution of Sulawesi Sea has been very uniform,and the salinity value of the whole sea is between 34.495-34.515‰(500 m)and 34.545-34.55‰(800 m).

Fig.10 shows the salinity distribution across the 123°E and 3°N sections of the Sulawesi Sea.Along these two salinity sections,from the surface to the depth of 150 m,the salinity value increases with the increase of depth,reaches the highest value(>34.7‰)at 150 m,and then gradually decreases to about 350 m,reducing to a very low value(≈34.4‰).From 400 m to 1 500 m,the salinity continues to increase with the increase of depth,but the increase range is slightly smaller than that of the upper layer.The two sections also show the distribution characteristics of two low and one high in the vertical direction.The shallow near surface layer at 100 m is a low salinity area(<34.5‰),and the salinity difference in the south-north direction is small(Fig.10a),while the salinity difference in the east-west direction is relatively large.The near surface layer along the 3°N section shows the characteristics of low in the east and high in the west(Fig.10b).Another low salinity area is located in the water layer of about 250 m-450 m.In the thickness of nearly 200 m,the vertical distribution of salinity is relatively uniform,and the salinity value is between 34.4-34.5‰.The low salinity area spans the whole Sulawesi Sea from east to west,and is in the north of 3-8°N.For the water layer between 100 m-200 m,there is obviously a high salinity area(34.6-34.8‰),which is located in the sea east of the middle of Sulawesi Sea,with a range of about 121-127°E and 3-6°N.In the sea deeper than 500 m,the salinity distribution has basically tended to be uniform.

Fig.10 Climatic salinity profile distribution along 123°E longitude(a)and 3°N latitude(b)in Sulawesi Sea

3.4 Temperature-Salinity

The temperature-salinity relationship(θ-S diagram)obtained from Argo temperature and salinity profiles from January 2004 to December 2019 is shown in Fig.11.On the whole,the temperature and salinity of Sulawesi Sea are about 2.5 °C-30 °C and 33.2-35.1‰,and the potential density is between 20-27.8 kg/m3.When the potential density is less than 21 kg/m3,the corresponding potential temperature is greater than 25 °C and the salinity is not more than 34‰,which is characterized by high temperature and low salinity water.The potential density is 21-22 kg/m3,the potential temperature is lower than that of the surface layer,but it still maintains a relatively high temperature(>25 °C),and the salinity increases significantly,concentrated between 33.5-34.5‰.When the potential density is between 22 kg/m3and 25 kg/m3,the potential temperature remains in the range of 15 °C-25 °C,and the salinity further increases significantly,mostly in the range of 34.0-35.1‰,showing the characteristics of high temperature and high salinity water.When the potential density is between 25k g/m3and 27 kg/m3,the temperature and salinity of the decrease sharply.At the depth where the potential density is greater than 26 kg/m3,the potential temperature value is lower than 10 °C,and the salinity value is less than 34.6‰.Compared with the upper layer,the deep layer θ-S point converges obviously,and thechanges of horizontal temperature and salinity become smaller obviously.When the potential density is greater than 27 kg/m3,the potential temperature continues to decrease below 5 °C,but the salinity(≈35.5‰)increases with depth,θ-S point converges further,and the horizontal distribution of temperature and salinity tends to be uniform,showing the characteristics of bottom water with low temperature and high salinity as a whole.

Fig.11 Sulawesi sea θ-S point graph

4 Conclusion

The Sulawesi Sea is an important maritime area surrounded by islands that connects southeast Asian countries to the Pacific and Indian Ocean.Based on the Argo temperature and salinity observations from 2004 to 2019 and the XBT temperature profile data obtained during the Chinese Antarctic scientific expedition,this paper analyzes and discusses the temperature and salinity environmental characteristics of 0 m-1 500 m in the sea.

(a)At the depth of 50 m-150 m,there is an obvious thermocline.The multi-year average depth of the upper boundary of the thermocline calculated by the maximum angle method is about 110 m,which is no more than 90 m in summer(July to September),which is obviously shallower than that in winter(January to March).Taking the thermocline as the boundary,the temperature above the thermocline gradually increases from northeast to southwest,while the deep high temperature area of 300 m moves northward obviously,showing a distribution trend of high in northwest and low in southeast.This seems to be related to the topography and current field of the sea:the surface water with high latitude and low temperature in the Northwest Pacific flows into the Sulawesi Sea along Mindanao lsland,and flows out of the southwest Makassar Strait along the South Bank of the Balabac Strait in the sea,roughly in the northeast southwest direction.Due to mixing,the shallow temperature distribution at 150 m in the sea also presents a trend of low in northeast and high in southwest.Moreover,the water depth at the east boundary of Sulawesi Sea is about 500 m-1 000 m,while the water depth in most of its internal sea areas is more than 5 000 m.Therefore,under the thermocline,the effect of water exchange in the Western Pacific is weakened,and the temperature is obviously different from that in the surface.

(b)Similar to the temperature,the surface salinity of Sulawesi Sea is about 33.5-34.0‰ due to the influence of current and topography.The salinity of 0 m-150 m increases with the increase of depth,up to more than 34.7‰ at about 150 m,and then gradually decreases to about 350 m with the increase of depth.At 400 m-1 500 m,the salinity increases gradually,but the increase range is slightly smaller than that of the upper layer.From the surface layer to 1 500 m deep layer,the salinity presents the distribution characteristics of two low and one high,while in the near surface layer(shallow at 150 m),the salinity level shows the trend of low in the southwest and high in the middle and north.

(c)Compared with the vertical variation characteristics,the horizontal temperature and salinity in the Sulawesi Sea area have little change,and with the increase of depth,the horizontal gradient gradually decreases,and the temperature and salinity distribution tends to be uniform up to 500 m deep.The temperature and salinity of the whole sea are about 2.5 °C-30 °C and 33.2-35.1‰ respectively.According to the potential density,from the surface to the bottom,it is shown as high temperature low salinity water,high temperature high salinity water,and low temperature high salinity water.

Acknowledgments

We thank China Argo Real-time Data Center for providing valuable information for this article,and the data processing group of Polar Research lnstitute of China for its contribution to the XBT quality control of this article.